The work proposed in this application is intended to increase our knowledge of how the veterbrate somite pattern is generated and maintained. Specifically it is proposed to use embryo culture and grafting techniques to exchange portions of the unsegmented somite mesoderm between embryos of the chicken and the Japanese quail. These exchanges will be carried out in such a way as to create disharmonies between the somite pattern specified by the host embryo and the somite pattern that would have been formed by the grafted mesoderm if it had been left in place. Through the use of histological analysis and time-lapse cinemicrography, it will be determined if the host embryos have the ability to regulate the number and time of formation of the graft somites. The information derived from this study is expected to aid in our eventual understanding of the morphogenesis and the teratogenesis of the vertebral column.